Defining tooth-moving appliances computationally

ABSTRACT

Methods and corresponding apparatus for segmenting an orthodontic treatment path into clinically appropriate substeps for repositioning the teeth of a patient include providing a digital finite element model of the shape and material of each of a sequence of appliances to be applied to a patient; providing a digital finite element model of the teeth and related mouth tissue of the patient; computing the actual effect of the appliances on the teeth by analyzing the finite elements models computationally; and evaluating the effect against clinical constraints. The appliances can be braces, polymeric shells, or other forms of orthodontic appliance.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.10/228,885, filed Aug. 26, 2002, now U.S. Pat. No. 6,682,346, which wasa continuation of U.S. application Ser. No. 09/169,034, filed Oct. 8,1998, now U.S. Pat. No. 6,471,511.

This application is related to commonly-owned U.S. application Ser. No.09/686,190, filed Oct. 10, 2000, and U.S. application Ser. No.09/169,036, filed Oct. 8, 1998, now U.S. Pat. No. 6,450,807, the fulldisclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to computational orthodontics.

In orthodontic treatment, a patient's teeth are moved from an initial toa final position using any of a variety of appliances. An applianceexerts force on the teeth by which one or more of them are moved or heldin place, as appropriate to the stage of treatment.

BRIEF SUMMARY OF THE INVENTION

The present invention provides methods and apparatus for definingappliance configurations at the steps of a process of repositioningteeth from an initial tooth arrangement to a final tooth arrangement.The invention can operate to define how repositioning is accomplished bya series of appliances or by a series of adjustments to appliancesconfigured to reposition individual teeth incrementally. The inventioncan be applied advantageously to specify a series of appliances formedas polymeric shells having the tooth-receiving cavities, that is, shellsof the kind described in the above-mentioned U.S. application Ser. No.09/169,276, filed Oct. 8, 1998 now abandoned.

A patient's teeth are repositioned from an initial tooth arrangement toa final tooth arrangement by making a series of incremental positionadjustments using appliances specified in accordance with the invention.In one implementation, the invention is used to specify shapes for theabove-mentioned polymeric shell appliances. The first appliance of aseries will have a geometry selected to reposition the teeth from theinitial tooth arrangement to a first intermediate arrangement. Theappliance is intended to be worn until the first intermediatearrangement is approached or achieved, and then one or more additional(intermediate) appliances are successively placed on the teeth. Thefinal appliance has a geometry selected to progressively repositionteeth from the last intermediate arrangement to a desired final tootharrangement.

The invention specifies the appliances so that they apply an acceptablelevel of force, cause discomfort only within acceptable bounds, andachieve the desired increment of tooth repositioning in an acceptableperiod of time. The invention can be implemented to interact with otherparts of a computational orthodontic system, and in particular tointeract with a path definition module that calculates the paths takenby teeth as they are repositioned during treatment.

In general, in one aspect, the invention provides methods andcorresponding apparatus for segmenting an orthodontic treatment pathinto clinically appropriate substeps for repositioning the teeth of apatient. The methods include providing a digital finite element model ofthe shape and material of each of a sequence of appliances to be appliedto a patient; providing a digital finite element model of the teeth andrelated mouth tissue of the patient; computing the actual effect of theappliances on the teeth by analyzing the finite elements modelscomputationally; and evaluating the effect against clinical constraints.Advantageous implementations can include one or more of the followingfeatures. The appliances can be braces, including brackets andarchwires, polymeric shells, including shells manufactured by stereolithography, retainers, or other forms of orthodontic appliance.Implementations can include comparing the actual effect of theappliances with an intended effect of the appliances; and identifying anappliance as an unsatisfactory appliance if the actual effect of theappliance is more than a threshold different from the intended effect ofthe appliance and modifying a model of the unsatisfactory applianceaccording to the results of the comparison. The model and resultingappliance can be modified by modifying the shape of the unsatisfactoryappliance, by adding a dimple, by adding material to cause anovercorrection of tooth position, by adding a ridge of material toincrease stiffness, by adding a rim of material along a gumline toincrease stiffness, by removing material to reduce stiffness, or byredefining the shape to be a shape defined by the complement of thedifference between the intended effect and the actual effect of theunsatisfactory appliance. The clinical constraints can include a maximumrate of displacement of a tooth, a maximum force on a tooth, and adesired end position of a tooth. The maximum force can be a linear forceor a torsional force. The maximum rate of displacement can be a linearor a angular rate of displacement. The apparatus of the invention can beimplemented as a system, or it can be implemented as a computer programproduct, tangibly stored on a computer-readable medium, havinginstructions operable to cause a computer to perform the steps of themethod of the invention.

Among the advantages of the invention are one or more of the following.Appliances specified in accordance with the invention apply no more thanorthodontically acceptable levels of force, cause no more than anacceptable amount of patient discomfort, and achieve the desiredincrement of tooth repositioning in an acceptable period of time. Theinvention can be used to augment a computational or manual process fordefining tooth paths in orthodontic treatment by confirming thatproposed paths can be achieved by the appliance under consideration andwithin user-selectable constraints of good orthodontic practice. Use ofthe invention to design aligners allows the designer (human orautomated) to finely tune the performance of the aligners with respectto particular constraints. Also, more precise orthodontic control overthe effect of the aligners can be achieved and their behavior can bebetter predicted than would otherwise be the case. In addition,computationally defining the aligner geometry facilitates direct alignermanufacturing under numerical control.

The details of one or more embodiments of the invention are set forth inthe

accompanying drawings and the description below. Other features andadvantages of the invention will become apparent from the description,the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of a process of specifying a course of treatmentincluding a subprocess for calculating aligner shapes in accordance withthe invention.

FIG. 2 is a flowchart of a process for calculating aligner shapes.

FIG. 3 is a flowchart of a subprocess for creating finite elementmodels.

FIG. 4 is a flowchart of a subprocess for computing aligner changes.

FIG. 5A is a flowchart of a subprocess for calculating changes inaligner shape.

FIG. 5B is a flowchart of a subprocess for calculating changes inaligner shape.

FIG. 5C is a flowchart of a subprocess for calculating changes inaligner shape.

FIG. 5D is a schematic illustrating the operation of the subprocess ofFIG. 5B.

FIG. 6 is a flowchart of a process for computing shapes for sets ofaligners.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, systems and methods are provided for definingappliance configurations or changes to appliance configurations forincrementally moving teeth. The tooth movements will be those normallyassociated with orthodontic treatment, including translation in allthree orthogonal directions relative to a vertical centerline, rotationof the tooth centerline in the two orthodontic directions (“rootangulation” and “torque”), as well as rotation about the centerline.

FIG. 1 illustrates the general flow of an exemplary process 100 fordefining and generating repositioning appliances for orthodontictreatment of a patient. The process 100 includes the methods, and issuitable for the apparatus, of the present invention, as will bedescribed. The computational steps of the process are advantageouslyimplemented as computer program modules for execution on one or moreconventional digital computers.

As an initial step, a mold or a scan of patient's teeth or mouth tissueis acquired (110). This step generally involves taking casts of thepatient's teeth and gums, and may also involve taking wax bites, directcontact scanning, x-ray imaging, tomographic imaging, sonographicimaging, and other techniques for obtaining information about theposition and structure of the teeth, jaws, gums and otherorthodontically relevant tissue. From the data so obtained, a digitaldata set is derived that represents the initial (that is, pretreatment)arrangement of the patient's teeth and other tissues.

The initial digital data set, which may include both raw data fromscanning operations and data representing surface models derived fromthe raw data, is processed to segment the tissue constituents from eachother (step 120). In particular, in this step, data structures thatdigitally represent individual tooth crowns are produced.Advantageously, digital models of entire teeth are produced, includingmeasured or extrapolated hidden surfaces and root structures.

The desired final position of the teeth—that is, the desired andintended end result of orthodontic treatment—can be received from aclinician in the form of a prescription, can be calculated from basicorthodontic principles, or can be extrapolated computationally from aclinical prescription (step 130). With a specification of the desiredfinal positions of the teeth and a digital representation of the teeththemselves, the final position and surface geometry of each tooth can bespecified (step 140) to form a complete model of the teeth at thedesired end of treatment. Generally, in this step, the position of everytooth is specified. The result of this step is a set of digital datastructures that represents an orthodontically correct repositioning ofthe modeled teeth relative to presumed-stable tissue. The teeth andtissue are both represented as digital data.

Having both a beginning position and a final position for each tooth,the process next defines a tooth path for the motion of each tooth. Thetooth paths are optimized in the aggregate so that the teeth are movedin the quickest fashion with the least amount of round-tripping to bringthe teeth from their initial positions to their desired final positions.(Round-tripping is any motion of a tooth in any direction other thandirectly toward the desired final position. Round-tripping is sometimesnecessary to allow teeth to move past each other.) The tooth paths aresegmented. The segments are calculated so that each tooth's motionwithin a segment stays within threshold limits of linear and rotationaltranslation. In this way, the end points of each path segment canconstitute a clinically viable repositioning, and the aggregate ofsegment end points constitute a clinically viable sequence of toothpositions, so that moving from one point to the next in the sequencedoes not result in a collision of teeth.

The threshold limits of linear and rotational translation areinitialized, in one implementation, with default values based on thenature of the appliance to be used. More individually tailored limitvalues can be calculated using patient-specific data. The limit valuescan also be updated based on the result of an appliance-calculation(step 170, described later), which may determine that at one or morepoints along one or more tooth paths, the forces that can be generatedby the appliance on the then-existing configuration of teeth and tissueis incapable of effecting the repositioning that is represented by oneor more tooth path segments. With this information, the subprocessdefining segmented paths (step 150) can recalculate the paths or theaffected subpaths.

At various stages of the process, and in particular after the segmentedpaths have been defined, the process can, and generally will, interactwith a clinician responsible for the treatment of the patient (step160). Clinician interaction can be implemented using a client processprogrammed to receive tooth positions and models, as well as pathinformation from a server computer or process in which other steps ofprocess 100 are implemented. The client process is advantageouslyprogrammed to allow the clinician to display an animation of thepositions and paths and to allow the clinician to reset the finalpositions of one or more of the teeth and to specify constraints to beapplied to the segmented paths. If the clinician makes any such changes,the subprocess of defining segmented paths (step 150) is performedagain.

The segmented tooth paths and associated tooth position data are used tocalculate clinically acceptable appliance configurations (or successivechanges in appliance configuration) that will move the teeth on thedefined treatment path in the steps specified by the path segments (step170). Each appliance configuration represents a step along the treatmentpath for the patient. The steps are defined and calculated so that eachdiscrete position can follow by straight-line tooth movement or simplerotation from the tooth positions achieved by the preceding discretestep and so that the amount of repositioning required at each stepinvolves an orthodontically optimal amount of force on the patient'sdentition. As with the path definition step, this appliance calculationstep can include interactions and even iterative interactions with theclinician (step 160). The operation of a process step 200 implementingthis step will be described more fully below.

Having calculated appliance definitions, the process 100 can proceed tothe manufacturing step (step 180) in which appliances defined by theprocess are manufactured, or electronic or printed information isproduced that can be used by a manual or automated process to defineappliance configurations or changes to appliance configurations.

FIG. 2 illustrates a process 200 implementing the appliance-calculationstep (FIG. 1, step 170) for polymeric shell aligners of the kinddescribed in above-mentioned patent application Ser. No. 09/169,276.Inputs to the process include an initial aligner shape 202, variouscontrol parameters 204, and a desired end configuration for the teeth atthe end of the current treatment path segment 206. Other inputs includedigital models of the teeth in position in the jaw, models of the jawtissue, and specifications of an initial aligner shape and of thealigner material. Using the input data, the process creates a finiteelement model of the aligner, teeth and tissue, with the aligner inplace on the teeth (step 210). Next, the process applies a finiteelement analysis to the composite finite element model of aligner, teethand tissue (step 220). The analysis runs until an exit condition isreached, at which time the process evaluates whether the teeth havereached the desired end position for the current path segment, or aposition sufficiently close to the desired end position (step 230). Ifan acceptable end position is not reached by the teeth, the processcalculates a new candidate aligner shape (step 240). If an acceptableend position is reached, the motions of the teeth calculated by thefinite elements analysis are evaluated to determine whether they areorthodontically acceptable (step 232). If they are not, the process alsoproceeds to calculate a new candidate aligner shape (step 240). If themotions are orthodontically acceptable and the teeth have reached anacceptable position, the current aligner shape is compared to thepreviously calculated aligner shapes. If the current shape is the bestsolution so far (decision step 250), it is saved as the best candidateso far (step 260). If not, it is saved in an optional step as a possibleintermediate result (step 252). If the current aligner shape is the bestcandidate so far, the process determines whether it is good enough to beaccepted (decision step 270). If it is, the process exits. Otherwise,the process continues and calculates another candidate shape (step 240)for analysis.

The finite element models can be created using computer programapplication software available from a variety of vendors. For creatingsolid geometry models, computer aided engineering (CAE) or computeraided design (CAD) programs can be used, such as the AutoCAD® softwareproducts available from Autodesk, Inc., of San Rafael, Calif. Forcreating finite element models and analyzing them, program products froma number of vendors can be used, including the PolyFEM product availablefrom CADSI of Coralville, Iowa, the Pro/Mechanica simulation softwareavailable from Parametric Technology Corporation of Waltham,Massachusetts, the I-DEAS design software products available fromStructural Dynamics Research Corporation (SDRC) of Cincinnati, Ohio, andthe MSC/NASTRAN product available from MacNeal-Schwendler Corporation ofLos Angeles, Calif.

FIG. 3 shows a process 300 of creating a finite element model that canbe used to perform step 210 of the process 200 (FIG. 2). Input to themodel creation process 300 includes input data 302 describing the teethand tissues and input data 304 describing the aligner. The input datadescribing the teeth 302 include the digital models of the teeth;digital models of rigid tissue structures, if available; shape andviscosity specifications for a highly viscous fluid modeling thesubstrate tissue in which the teeth are embedded and to which the teethare connected, in the absence of specific models of those tissues; andboundary conditions specifying the immovable boundaries of the modelelements. In one implementation, the model elements include only modelsof the teeth, a model of a highly viscous embedding substrate fluid, andboundary conditions that define, in effect, a rigid container in whichthe modeled fluid is held.

A finite element model of the initial configuration of the teeth andtissue is created (step 310) and optionally cached for reuse in lateriterations of the process (step 320). As was done with the teeth andtissue, a finite element model is created of the polymeric shell aligner(step 330). The input data for this model includes data specifying thematerial of which the aligner is made and the shape of the aligner (datainput 304).

The model aligner is then computationally manipulated to place it overthe modeled teeth in the model jaw to create a composite model of anin-place aligner (step 340). Optionally, the forces required to deformthe aligner to fit over the teeth, including any hardware attached tothe teeth, are computed and used as a figure of merit in measuring theacceptability of the particular aligner configuration. In a simpleralternative, however, the aligner deformation is modeled by applyingenough force to its insides to make it large enough to fit over theteeth, placing the model aligner over the model teeth in the compositemodel, setting the conditions of the model teeth and tissue to beinfinitely rigid, and allowing the model aligner to relax into positionover the fixed teeth. The surfaces of the aligner and the teeth aremodeled to interact without friction at this stage, so that the alignermodel achieves the correct initial configuration over the model teethbefore finite element analysis is begun to find a solution to thecomposite model and compute the movement of the teeth under theinfluence of the distorted aligner.

FIG. 4 shows a process 400 for calculating the shape of a next alignerthat can be used in the aligner calculations, step 240 of process 200(FIG. 2). A variety of inputs are used to calculate the next candidatealigner shape. These include inputs 402 of data generated by the finiteelement analysis solution of the composite model and data 404 defined bythe current tooth path. The data 402 derived from the finite elementanalysis includes the amount of real elapsed time over which thesimulated repositioning of the teeth took place; the actual end toothpositions calculated by the analysis; the maximum linear and torsionalforce applied to each tooth; the maximum linear and angular velocity ofeach tooth. From the input path information, the input data 404 includesthe initial tooth positions for the current path segment, the desiredtooth positions at the end of the current path segment, the maximumallowable displacement velocity for each tooth, and the maximumallowable force of each kind for each tooth.

If a previously evaluated aligner was found to violate one or moreconstraints, additional input data 406 can optionally be used by theprocess 400. This data 406 can include information identifying theconstraints violated by, and any identified suboptimal performance of,the previously evaluated aligner.

Having received the initial input data (step 420), the process iteratesover the movable teeth in the model. (Some of the teeth may beidentified as, and constrained to be, immobile.) If the end position anddynamics of motion of the currently selected tooth by the previouslyselected aligner is acceptable (“yes” branch of decision step 440), theprocess continues by selecting for consideration a next tooth (step 430)until all teeth have been considered (“done” branch from step 430 tostep 470). Otherwise (“no” branch from step 440), a change in thealigner is calculated in the region of the currently selected tooth(step 450). The process then moves back to select the next current tooth(step 430) as has been described.

When all of the teeth have been considered, the aggregate changes madeto the aligner are evaluated against previously defined constraints(step 470), examples of which have already been mentioned. Constraintscan be defined with reference to a variety of further considerations,such as manufacturability. For example, constraints can be defined toset a maximum or minimum thickness of the aligner material, or to set amaximum or minimum coverage of the aligner over the crowns of the teeth.If the aligner constraints are satisfied, the changes are applied todefine a new aligner shape (step 490). Otherwise, the changes to thealigner are revised to satisfy the constraints (step 480), and therevised changes are applied to define the new aligner shape (step 490).

FIG. 5A illustrates one implementation of the step of computing analigner change in a region of a current tooth (step 450). In thisimplementation, a rule-based inference engine 456 is used to process theinput data previously described (input 454) and a set of rules 452 a-452 n in a rule base of rules 452. The inference engine 456 and therules 452 define a production system which, when applied to the factualinput data, produces a set of output conclusions that specify thechanges to be made to the aligner in the region of the current tooth(output 458).

Rules 452 have the conventional two-part form: an if-part defining acondition and a then-part defining a conclusion or action that isasserted if the condition is satisfied. Conditions can be simple or theycan be complex conjunctions or disjunctions of multiple assertions. Anexemplary set of rules, which defines changes to be made to the aligner,includes the following: if the motion of the tooth is too slow, adddriving material to the aligner opposite the desired direction ofmotion; if the motion of the tooth is too slow, add driving material toovercorrect the position of the tooth; if the tooth is too far short ofthe desired end position, add material to overcorrect; if the tooth hasbeen moved too far past the desired end position, add material tostiffen the aligner where the tooth moves to meet it; if a maximumamount of driving material has been added, add material to overcorrectthe repositioning of the tooth and do not add driving material; if themotion of the tooth is in a direction other than the desired direction,remove and add material so as to redirect the tooth.

In an alternative embodiment, illustrated in FIGS. 5B and 5C, anabsolute configuration of the aligner is computed, rather than anincremental difference. As shown in FIG. 5B, a process 460 computes anabsolute configuration for an aligner in a region of a current tooth.Using input data that has already been described, the process computesthe difference between the desired end position and the achieved endposition of the current tooth (462). Using the intersection of the toothcenter line with the level of the gum tissue as the point of reference,the process computes the complement of the difference in all six degreesof freedom of motion, namely three degrees of translation and threedegrees of rotation (step 464). Next, the model tooth is displaced fromits desired end position by the amounts of the complement differences(step 466), which is illustrated in FIG. 5D.

FIG. 5D shows a planar view of an illustrative model aligner 60 over anillustrative model tooth 62. The tooth is in its desired end positionand the aligner shape is defined by the tooth in this end position. Theactual motion of the tooth calculated by the finite element analysis isillustrated as placing the tooth in position 64 rather than in thedesired position 62. A complement of the computed end position isillustrated as position 66. The next step of process 460 (FIG. 5B)defines the aligner in the region of the current tooth in this iterationof the process by the position of the displaced model tooth (step 468)calculated in the preceding step (466). This computed alignerconfiguration in the region of the current tooth is illustrated in FIG.5D as shape 68 which is defined by the repositioned model tooth inposition 66.

A further step in process 460, which can also be implemented as a rule452 (FIG. 5A), is shown in FIG. 5C. To move the current tooth in thedirection of its central axis, the size of the model tooth defining thatregion of the aligner, or the amount of room allowed in the aligner forthe tooth, is made smaller in the area away from which the process hasdecided to move the tooth (step 465).

As shown in FIG. 6, the process 200 of computing the shape for analigner for a step in a treatment path is one step in an overall process600 of computing the shapes of a series of aligners. This overallprocess 600 begins with an initialization step 602 in which initialdata, control and constraint values are obtained.

When an aligner configuration has been found for each step or segment ofthe treatment path (step 604), the overall process 600 determineswhether all of the aligners are acceptable (step 606). If they are, theprocess exits and is complete. Otherwise, the process optionallyundertakes a set of steps 610 in an attempt to calculate a set ofacceptable aligners. First, one or more of the constraints on thealigners is relaxed (step 612). Then, for each path segment with anunacceptable aligner, the process 200 of shaping an aligner is performedwith the new constraints (step 614). If all the aligners are nowacceptable, the overall process 600 exits (step 616).

Aligners may be unacceptable for a variety of reasons, some of which arehandled by the overall process. For example, if any impossible movementswere required (decision step 620), that is, if the shape calculationprocess 200 was required to effect a motion for which no rule oradjustment was available, the process 600 proceeds to execute a modulethat calculates the configuration of a hardware attachment to thesubject tooth to which forces can be applied to effect the requiredmotion (step 640). Because adding hardware can have an effect that ismore than local, when hardware is added to the model, the outer loop ofthe overall process 600 is executed again (step 642).

If no impossible movements were required (“no” branch from step 620),the process transfers control to a path definition process (such as step150, FIG. 1) to redefine those parts of the treatment path havingunacceptable aligners (step 630). This step can include both changingthe increments of tooth motion, i.e., changing the segmentation, on thetreatment path, changing the path followed by one or more teeth in thetreatment path, or both. After the treatment path has been redefined,the outer loop of the overall process is executed again (step 632). Therecalculation is advantageously limited to recalculating only thosealigners on the redefined portions of the treatment path. If all thealigners are now acceptable, the overall process exits (step 634). Ifunacceptable aligners still remain, the overall process can be repeateduntil an acceptable set of aligners is found or an iteration limit isexceeded (step 650). At this point, as well as at other point in theprocesses that are described in this specification, such as at thecomputation of additional hardware (step 640), the process can interactwith a human operator, such as a clinician or technician, to requestassistance (step 652). Assistance that an operator provides can includedefining or selecting suitable attachments to be attached to a tooth ora bone, defining an added elastic element to provide a needed force forone or more segments of the treatment path, suggesting an alteration tothe treatment path, either in the motion path of a tooth or in thesegmentation of the treatment path, and approving a deviation from orrelaxation of an operative constraint.

As was mentioned above, the overall process 600 is defined andparameterized by various items of input data (step 602). In oneimplementation, this initializing and defining data includes thefollowing items: an iteration limit for the outer loop of the overallprocess; specification of figures of merit that are calculated todetermine whether an aligner is good enough (see FIG. 2, step 270); aspecification of the aligner material; a specification of theconstraints that the shape or configuration of an aligner must satisfyto be acceptable; a specification of the forces and positioning motionsand velocities that are orthodontically acceptable; an initial treatmentpath, which includes the motion path for each tooth and a segmentationof the treatment path into segments, each segment to be accomplished byone aligner, a specification of the shapes and positions of any anchorsinstalled on the teeth or otherwise; and a specification of a model forthe jaw bone and other tissues in or on which the teeth are situated (inthe implementation being described, this model consists of a model of aviscous substrate fluid in which the teeth are embedded and which hasboundary conditions that essentially define a container for the fluid).

Optionally, other features are added to the tooth model data sets toproduce desired features in the aligners. For example, it may bedesirable to add digital wax patches to define cavities or recesses tomaintain a space between the aligner and particular regions of the teethor jaw. It may also be desirable to add digital wax patches to definecorrugated or other structural forms to create regions having particularstiffness or other structural properties. In manufacturing processesthat rely on generation of positive models to produce the repositioningappliance, adding a wax patch to the digital model will generate apositive mold that has the same added wax patch geometry. This can bedone globally in defining the base shape of the aligners or in thecalculation of particular aligner shapes. One feature that can be addedis a rim around the gumline, which can be produced by adding a digitalmodel wire at the gumline of the digital model teeth from which thealigner is manufactured. When an aligner is manufactured by pressurefitting polymeric material over a positive physical model of the digitalteeth, the wire along the gumlines causes the aligner to have a rimaround it providing additional stiffness along the gumline.

In another optional manufacturing technique, two sheets of material arepressure fit over the positive tooth model, where one of the sheets iscut along the apex arch of the aligner and the other is overlaid on top.This provides a double thickness of aligner material along the verticalwalls of the teeth.

The changes that can be made to the design of an aligner are constrainedby the manufacturing technique that will be used to produce it. Forexample, if the aligner will be made by pressure fitting a polymericsheet over a positive model, the thickness of the aligner is determinedby the thickness of the sheet. As a consequence, the system willgenerally adjust the performance of the aligner by changing theorientation of the model teeth, the sizes of parts of the model teeth,the position and selection of attachments, and the addition or removalof material (e.g., adding wires or creating dimples) to change thestructure of the aligner. The system can optionally adjust the alignerby specifying that one or more of the aligners are to be made of a sheetof a thickness other than the standard one, to provide more or lessforce to the teeth. On the other hand, if the aligner will be made by astereo lithography process, the thickness of the aligner can be variedlocally, and structural features such as rims, dimples, and corrugationscan be added without modifying the digital model of the teeth.

The system can also be used to model the effects of more traditionalappliances such as retainers and braces and therefore be used togenerate optimal designs and treatment programs for particular patients.

The data processing aspects of the invention can be implemented indigital electronic circuitry, or in computer hardware, firmware,software, or in combinations of them. Data processing apparatus of theinvention can be implemented in a computer program product tangiblyembodied in a machine-readable storage device for execution by aprogrammable processor; and data processing method steps of theinvention can be performed by a programmable processor executing aprogram of instructions to perform functions of the invention byoperating on input data and generating output. The data processingaspects of the invention can be implemented advantageously in one ormore computer programs that are executable on a programmable systemincluding at least one programmable processor coupled to receive dataand instructions from and to transmit data and instructions to a datastorage system, at least one input device, and at least one outputdevice. Each computer program can be implemented in a high-levelprocedural or object oriented programming language, or in assembly ormachine language, if desired; and, in any case, the language can be acompiled or interpreted language. Suitable processors include, by way ofexample, both general and special purpose microprocessors. Generally, aprocessor will receive instructions and data from a read-only memoryand/or a random access memory. Storage devices suitable for tangiblyembodying computer program instructions and data include all forms ofnonvolatile memory, including by way of example semiconductor memorydevices, such as EPROM, EEPROM, and flash memory devices; magnetic diskssuch as internal hard disks and removable disks; magneto-optical disks;and CD-ROM disks. Any of the foregoing can be supplemented by, orincorporated in, ASICs (application-specific integrated circuits).

To provide for interaction with a user, the invention can be implementedusing a computer system having a display device such as a monitor or LCD(liquid crystal display) screen for displaying information to the userand input devices by which the user can provide input to the computersystem such as a keyboard, a two-dimensional pointing device such as amouse or a trackball, or a three-dimensional pointing device such as adata glove or a gyroscopic mouse. The computer system can be programmedto provide a graphical user interface through which computer programsinteract with users. The computer system can be programmed to provide avirtual reality, three-dimensional display interface.

The invention has been described in terms of particular embodiments.Other embodiments are within the scope of the following claims. Forexample, the steps of the invention can be performed in a differentorder and still achieve desirable results.

What is claimed is:
 1. A computer-implemented method for segmenting anorthodontic treatment path into clinically appropriate substeps forrepositioning the teeth of a patient, comprising: providing a digitalmodel of the shape and material of each of a sequence of appliances tobe applied to a scanned model of the teeth; computing the actual effectof the appliances on the teeth by performing finite element analysis oneach of said models; evaluating the effect against clinical constraints;and generating data sets corresponding to a plurality of applianceshaving geometries selected to progressively reposition the teeth,wherein the appliances comprise polymeric shells having cavities andwherein the cavities of successive shells have different geometriesshaped to receive and resiliently reposition teeth from one arrangementto a successive arrangement.
 2. A method as in claim 1, furthercomprising fabricating individual polymeric shell appliances based onindividual ones of the data sets.
 3. The method of claim 2, wherein theappliances are manufactured by fitting polymeric sheets over positivemodels corresponding to the data sets.
 4. The method of claim 1, whereinthe sequence of appliances includes a sequence of polymeric shellsmanufactured by stereo lithography from digital models.
 5. The method ofclaim 1, further comprising: comparing the actual effect of theappliances with an intended effect of the appliances; identifying anappliance as an unsatisfactory appliance if the actual effect of theappliance is more than a threshold different from the intended effect ofthe appliance; and modifying a model of the unsatisfactory applianceaccording to the results of the comparison.
 6. The method of claim 5,wherein the model of the unsatisfactory appliance is modified bymodifying the shape of the unsatisfactory appliance.
 7. The method ofclaim 6, wherein the shape of the unsatisfactory appliance is modifiedby adding a dimple.
 8. The method of claim 6, wherein the shape of theunsatisfactory appliance is modified by adding material to cause anovercorrection of tooth position.
 9. The method of claim 6, wherein theshape of the unsatisfactory appliance is modified by adding a ridge ofmaterial to increase stiffness.
 10. The method of claim 6, wherein theshape of the unsatisfactory appliance is modified by adding a rim ofmaterial along a gumline to increase stiffness.
 11. The method of claim6, wherein the shape of the unsatisfactory appliance is modified byremoving material to reduce stiffness.
 12. The method of claim 5,wherein the unsatisfactory appliance is redefined to have a shapedefined by the complement of the difference between the intended effectand the actual effect of the unsatisfactory appliance.
 13. The method ofclaim 1, wherein the clinical constraints include a maximum rate ofdisplacement of a tooth, a maximum force on a tooth, and a desired endposition of a tooth.
 14. The method of claim 13, wherein the maximumforce is a linear force or a torsional force.
 15. The method of claim13, wherein the maximum rate of displacement is a linear or a angularrate of displacement.